At last week's Guadec meeting, Kristian Rietveld delivered the GTK+ "state of the union" report. GTK+ is the multi-platform toolkit behind a number of popular applications and, perhaps most well known, the Gnome Desktop environment for Linux. Read the full report here.

So, do you think is feasible to write the new GTK+ stuff JUST in Vala?

Vala would be in charge of all the plumbing and the developer would not need about anything (that's the rational behind Vala, of course)

Basically it should be possible, but Vala has still bugs and the syntax is still changing sometimes, it's not a final version yet. GTK developers would have to tightly cooperate with the Vala developers. But as I understand GTK+ 3.x is not intended as a complete rewrite, but as a major refactoring and extension. In my opinion it would be reasonable to implement new widgets and components in Vala and let the others be plain C until they get replaced at some time. GObject/C and Vala classes happily work together as they are the same.